Developments on gelling algal galactans, their structure and physico-chemistry

The wide uses of the algal galactans agar, agarose and carrageenans are based on their unique properties to form strong gels in aqueous solutions. These gels result from peculiar regular chemical structures, specific ordered molecular conformations and aggregations. In recent years, new methodologies and instruments have provided a more accurate view of the relationships between the chemical structure and the gelling characteristics of these complex hybrid and heterogeneous polysaccharides. Methanolysis and reductive acid hydrolysis procedures coupled to different chromatographic separations allowing the quantitative determination of all the constituent sugars including the acid labile 3,6-anhydyrogalactose are particularly emphasised. Means of determining sugar linkages, substitutions and sequences using chemical, enzymatic and spectroscopic methods are also presented. Developments in multi- and low-angle laser-light diffusion detectors coupled to high performance size exclusion chromatography now render the determination of molecular weight and molecular weight distribution of these galactans more accessible. Such techniques also yield new information on the aggregate formation of these sulphated polysaccharides. These and other data question the existence of the generally assumed intertwined double helical conformations of these galactans during gel formation.